Transformer



Dec. 11, 1951 J. H. BYRIDGES I 2,577,733

TRANSFORMER Original Filed March 5, 1945 2 SHEETS-SHEET l Jo -m H md rides INVENTOR.

H S n'rroRNEY J. H- BRIDGES TRANSFORMER Original Filed March 5, 1945 2Si-IEETS-SHEET 2 INZgNTOR. I m1 Patented Dec. 11, 1951 TRANSFORMER JohnHerold Bridges, Fair-burn, 6a., minor to National Inven of New Jerseyions Corporation, a corporation Original application March 5, 1945,Serial No.

581,055. Divided and this application Novemher 1, 1946, Serial No.707,294; 11103119413 August 20, 1943 1 Claim. 1

My present application for patent is a division of my copendingapplication 581,055 of March 5, 1945, entitled Luminescent Tube Systemand Apparatus, now Patent 2,510,209 of June 6, 1950, which in turn is acontinuation in part of my copending application, Serial No. 448,471filed June 25, 1942, and entitled Luminescent Tube System, now Patent2,370,635 of March 6, 1945, and the invention relates to transformersand power units for powering fluorescent lighting systems and other tubeloads of negative resistance characteristics.

An important object of my invention is to providea transformer unitcharacterized by its small iron and copper requirement, its consequentlow weight, its compact, self-contained and unitary design, with smallspace requirements, its physical sturdiness and low first cost, both ofmaterials and of assembly, and, as well, by its low core loss and highefllciency.

Another object is to provide a new transformer of such construction thatdetrimental over-loading is effectively prevented at all times, in whichall necessity of protective fuses or other safety mechanisms is avoided,the entire construction being in full compliance with all requirementsof the Fire Underwriters.

Other objects in part will be obvious and in part pointed outhereinafter during the course of the following description.

My invention accordingly resides in the various elements and features ofconstruction, and in the combination of parts, the scope of theapplication of all of which is more fully set forth in the claims at theend of this specification.

In the drawings wherein I have disclosed several embodiments of myinvention which I prefer at present,

Figures 1 and 2 schematically illustrate two circuits embodying myinventive thought, while Figure 3 illustrates, in perspective, my newpower unit.

Throughout the drawings like reference characters indicate like parts.

In my earliest parent patent, it has been fully revealed how fluorescenttube lighting, over say the past decade, has met with widespreadacceptance throughout the arts, commerce, industry, and as well, inalmost all types of household utilization. The marked advance inemciency, and in reduced operational costs, the absence of heat and manyother characteristic advantages as contrasted with the Edisontype offilamentary lamp readily explains the warm re- 2 ception with which thistype of illumination has been received.

An important object of my invention, therefore, is to provide a newtransformer, which while avoiding in large measure the aforementioneddisadvantages and deficiencies, possesses the multiple advantages ofrequiring a minimum investment in copper and iron, and consequently isneat, sturdy, compact and self-contained, lending itself to readymounting on the reflector or other mounting of an associated fixture,and which displays high eificiency with good system power factor,quick-starting characteristics under cold weather operation, in thesubstantial absence of detrimental stroboscopic flicker, and which issubstantially fool-proof, no circuit protective auxiliaries beingrequired.

Referring now more particularly to the simplest embodiment of myinvention, attention is directed to Figure 1 of the drawings, in whichthere is shown a transformer with secondary coils inductively associatedwith the primary winding in ordinary transformer connection. A magneticcore, indicated generally at it. is here illustrated as being generallyof the shell type, having a central, longitudinally extending leg II,constituting an inner core portion, flanked in spaced relation, oneither side thereof, by outer longitudinally-extending legs l2 and I3,constituting outer core portions. Leg H is designed to accommodate,without saturation, the maximum magnetic flux developed by the primarywinding later to be described.

It is not essential that legs l2 and I3 be of like dimensions or shape,or that they be equally spaced from central leg Ii. Accommodation forany variation in such design can be accomplished by compensatingvariations in other features of design. However, for symmetry as well asfor many other reasons involving electrical and mechanical design, Iprefer to construct legs I2 and I3 of like configuration, and to spacethem equi-distant from central leg II. This gives a more readilybalancedcondition of magnetic flux.

Inasmuch as the flux from leg ll divides substantially equally betweenlegs I! and I3, it is sumcient that each of these have a cross-sectionalarea substantially half that of leg I I. End members or pieces I4, l4and l5, l5 serve to interconnect opposed ends of legs I I, l2, 13 so asto form a closed magnetic core, of the shell type. In the constructionshown, these in reality constitute end-leg extensions of substantiallyU- shaped core members, the yoke portions of which are constituted bylegs l2, l3, respectively. It is entirely satisfactory, however, toconstruct them in any desired suitable manner, the controlling criterionbeing that the legs are stamped from suitable sheet laminations, and aresubsequently assembled in the manner giving rise to the highest over-allefficiency. However, they be stamped and assembled, the legs II, I2, I3and end pieces I4, I4 and I5, I5 are constructed in stacks of lamina ofsuitable steel, displaying little residual ma netis'm. End pieces I4, IIand I5, I5,.for obvious reasons have cross-sectional areas approximatelythose of legs I2, I3.

Intermediate the lengths of legs II, I2, and I3, and extending eitherfrom leg II towards but short of legs I2, I3 respectively, or as shown,from legs I2 and I3 towards but short of leg II, are a plurality ofmagnetic shunts I6, I6 and I], I1. These shunts, of laminarconstruction, may be formed in any suitable manner. In the presentinstance, they are struck integrally from legs I2 and I3, but certainsavings in iron are had by employing laminated inserts wedged betweenthe windings. Moreover, it is preferred, as generally indicated inFigure 2, that the width of shunts I'I, I1 be substantially that of theshunts I6, I6. I find that shunts l1, l1 need only be about onehalf aswide as shunts I6, I6 for reasons dealt with hereinafter.

The shunts I6, I6 and I1, I1, terminating short of central leg II,provide included air-gaps GI, G2, G3 and G4 therebetween. In either theintegral or insert construction, however, it will be noticed thatair-gaps GI, G2 are shorter than air-gaps G3, G4, giving rise to greaterreluctance.

The purpose of this design will be developed at a later point. It issufficient here to note that these air-gaps are calibrated in accordancewith the particular load demands on the corresponding secondary windingslater to be described.

The magnetic core shunts I6, I6 and II, II,

together with the remaining elements of the magnetic core, define aplurality of compartments CI, C2 and C3, serving respectively to houseprimar winding I8 and secondary windings SI, S2. Since primary I8 servesto energize both secondaries SI, S2, only a single primary winding I8 isrequired. While this winding must be of substantially larger size wirethan would be required were separate primaries used, nevertheless asubstantial saving in copper results from the use of one primary windingonly. Moreover, the reduction in physical dimensions made possible bythe elimination of one primary winding employed in certain priorconstruction makes feasible a substantial decrease in the total ironcontent of the transformer. This is brought about by decreasing thelengths of the magnetic paths, and results in appreciable decrease inweight, with corresponding gain in compactness. A small, neat-appearingunit is thus made possible.

Primary I8 of step-up transformer I0 is therefore constructed of arelatively small number of turns of comparatively heavy gauge wire. Itis energized from any suitable source of alternating current energyindicated generally at I9. In my preferred embodiment this compriseseither a 110 volt or 220 volt line. Leads 28, 2| serve to establish thisconnection.

Paired fluorescent discharge tubes 22, 23 are included in circuit withsecondary windings SI, S2, respectively. These fluorescent tubes mayeither be especially designed for cold-cathode operation, in whichinstance they will likely be fashioned with a single, solid electrode ateach end'of the tube, or else the conventional hotcathodetube, nowreadily available on the market, can be adapted in comparatively simplemanner for entirely satisfactory operation under coldcathode conditions.The electrodes of these tubes are conditioned for such cold-cathodeoperation by installing a jumper across the paired terminals of thefilamentary electrode provided at each end of the tube. Theshort-circuit thus established ensures that all parts of the electrodeare at the same potential.

Tube 22 is connected by leads 24, 25 across the terminals of secondarySI, while tube 23 is energized through leads 28, 21 from secondary S2.Contributing to the elimination of stroboscopic effect, the relation oftube 23 to its secondary S2 is reversed with respect to the relation oftube 22 to secondary SI. That is, while the right terminal of tube 22 isconnected to the right terminal of secondary SI, it is the left terminalof tube 23 which is connected to the right terminal of secondary S2.

A power-factor correcting condenser 28 is provided in lead 26 betweentube 23 and secondary S2. It will be recalled that it is this secondary,in compartment C3, which is associated with the short shunts II, II. Thepurpose of this will be developed. The condenser 28 is chosen ofsufficient size and capacity effectively to restore system power factor,made lagging by the highly inductive nature of the transformer load, tosubstantially unity value. Additionally, the leading characteristicimparted by this condenser to its corresponding secondary circuiteffectively throws the tubes 22, 23 out of phase. One will ignite whilethe other is dark, and vice versa, so that detrimental stroboscopiceffect, evidenced as an undesirable flicker is substantially removed.

It will now be in order to describe the operation of my newconstruction. Let us assume that for a given half-cycle of primarycurrent, this current flow is such that a primary flux courses to theright in Figure 1, along central leg I I. At this time no arc has beenstruck across secondaries SI and S2, so that no limitative values areimposed on the coursing of flux due to saturation effects.

It is in order to digress momentarily at this point and call attentionto the fact that while the magnetic core which I disclose and prefer isof the shell type, with symmetrically constructed and disposed legs,nevertheless, as has already been pointed out, these legs may be ofasymmetrical construction and disposition. In point of fact, it isentirely possible to dispense entirely with one outer leg, increasingthe dimensions of the other outer leg, and resulting in a core-typeconstruction. I find somewhat better transformer performance withimproved wave-form however, when the shell-type transformer is employed.

The flux stream interlinks secondary SI, which at that time is underopen-circuit conditions, and generates a secondary voltage therein atthe right end of leg II this flux stream separates into twosubstantially equal branch streams. One of these courses up upper endmember It to leg I2, and across this to the left in Figure 1, and thendown upper end piece I5, back along the right through leg II,interlinking secondary S2, then under openl-acircuit conditions, andthence back to winding At II, the other branch stream courses downacross lower end piece I4, across leg I3 to the left in Figure 1, and uplower end piece to the left end of leg II. There it reunites with thestream first described, whereupon the combined stream courses to theright along le I I, back to the primary coil section.

Inasmuch as no current flows through secondary windings SI and S2 atthis time, no back or secondary magnetomotive force is developed in therespective secondaries, impeding the coursing of the primary fiux. Thus,no reluctance is interposed in the main magnetic paths, interlinking thesecondary windings. The primary flux courses unimpeded across them.Thus, at these times air-gaps GI, G2, G3 and G4, particularly thelastmentioned, have reluctances which are comparatively much greaterthan those of the main magnetic path. Accordingly, both during thecoursing of the main fiux stream across leg I I, and of the branchedstreams across legs I2, I3, respectively, there is no tendency for anysubstantial part of the fiux to course across the magnetic shunts I5, I6and IT, II. All of the flux is available for voltage build-up in thesecondary coil sections.

During the next succeeding half-cycle of primary current, the coursingof developed flux is Just the reverse of that described.

As has already been described in connection with the generation of fiuxduring the preceding half-cycle, the reluctance of the air-gaps GIthrough G4, inclusive, are such compared to the reluctance interposed bywindings. SI, S2 under open-circuit conditions that practically no fluxcourses the branch paths provided by shunts I8, I6 and H, IT. Thisreversal of fiux during each half-cycle, say 120 times per second forordinary 60 cycle current, ensures rapid build-up of voltage in theopen-circuit secondary coils. Voltage rises and falls cyclically inthese coils to a value substantially in excess of that required tostrike arcs across the corresponding tube loads. Finally, after thepassage of a number of cycles of current, one of the tubes isconditioned to start. That is, the stresses produced across the tubeelectrodes so excite the gas content of the tube as to ionize thelatter, and condition it for carrying an arc across from one electrodeto the other. The rapid reversal of these stresses, and the tremendousvalue thereof due to the high open-circuit voltages, quickly bringsabout this phenomenon. In the space of but a fraction of a second, then,the arc is struck across one of the tubes. This compares most favorablywith the earlier tube assemblies, in which a starting time of at leastsix or seven seconds is required, and even more during cold weatheroperation. I find that on the contrary, weather has little if any effectin the striking and maintenance of the arc across the tubes of my newsystem. I am satisfied that this advantageous operation is accountablein large measure to high open-circuit voltage.

For illustration, we will assume that it is tube 23 across which the arcfirst strikes initially. Of course, were it the tube 22 across which thearc is first established, the operation would be just the reverse ofthat described, as will be clearly apparent to those skilled in the art.We will further assume that at this moment, the half-cycle of primarycurrent is such that primary fiux courses to the right along leg II,from primary I8. As soon as the arc strikes across tube 23, a currentflow occurs across this tube. Because of the negative resistancecharacteristics of this tube load, this current may attain a substantialvalue. The secondary current induces a secondary or back magnetic fluxwhich links leg I I and courses in a direction opposite to the primaryflux, effectively bucking the same. On the other hand, unless adequateprotective devices are provided across the line, the secondary currentwill build up indefinitely, resulting in rapid failure or evendestruction of the tube or system, or both.

It is for this purpose, and to interpose effective and permanentcontrol, simple in nature, that the shunt leakage paths are provided.Whereas the leakage path interposed by air-gaps G3 and G4, of fixedmagnetic reluctance, were heretofore of substantially greater reluctancethan the main magnetic path or paths on opencircuit conditions, they arenow of substantially smaller reluctance than the main magnetic paths,across which the back magnetomotive forces hold sway, bucking theprimary flux.

To illustrate, let us trace the primary flux paths under theseconditions. Primary flux courses to the right along leg I I, fromprimary I8. It interlinks secondary SI, across'which no load has as yetbeen established. Separating at the right end of leg II, one branchcourses up, and the other down, the end pieces I4, I4. Since the twobranch paths are in parallel and are almost exactly alike, it will besufficient, for illustration, to describe but a single one of them. Thefiux courses up the upper end piece I4, to the left along leg I2.Because of the minimum reluctance interposed by coil section SI, littlefiux courses across air-gap GI.

When the primary flux comes opposite the region defining compartment G3,however, housing secondary S2, it is met and bucked by a substantialflow of reverse or secondary fiux. Choosing the path of leastreluctance, the greater part of the primary flux courses down acrossshunt I1 and air-gap G3 to leg I I, to the left of primary I8, andthence back to the right along this leg to primary I8. Thus, the greaterpart of the flux combines with the back magnetic flux in coursing downthis branch leakage path and across airgap G3, the secondary fiuxreturning the shortest path to secondary S2, and the primary fluxsimilarly returning by shortest path to primary I8.

The calibrated design of shunts I1, I! is such, relative to the intendedsecondary load, that only that quantity of primary flux will at thistime course to the left along leg I 2, down upper end piece I5, and tothe right along leg II, interlinking S2, as will be sufficient togenerate a voltage which will induce the required operating current fortube 23 under load conditions, that is, which an arc is establishedacross this tube. At this time, by far greater part of the primary fluxis short-circuited across the high-reluctance shunt path, directly backto primary I8.

During the next primary current half-cycle, the situation is just thereverse of that described.

Shortly after the arc has struck across tube 23, the full primary flux,interlinking secondary Si, conditions the latter for striking. This isall but a matter of a fraction of a second. As soon as the arc strikesacross tube 22, load conditions are established across this secondarycircuit, and current begins to fiow of substantial value. A secondarymagnetomotive force is developed, just as in the case of secondary S2,giving rise to a secondary magnetic flux of substantial importance. Thisis in a, direction which is the reverse of one which bucks the primaryfiux. It effectively prevents the interlinking of all the primary fiuxwith secondary SI. Only enough flux interlinks the turns of this windingto induce a voltage sufificient to provide required secondary currentacross tube 22 to maintain the established are. This phenomenon isoccasioned by the high leakage reaetance shunts I6, I6 and includedairgaps GI, G2.

In the embodiment of Figure l, a three-coil transformer has beendisclosed in which the secondaries and primary, are only inductivelyconnected, and are physically independent of each other. This, forconvenience, I term ordinary transformer connection. It is entirelyfeasible under many circumstances, however, and -from' an ecwmicstandpoint, oftentimes preferable, to employ autotransformer connectionsbetween the primary and secondary windings. When these connections areemployed, and the design is such that secondary maximum voltages do notsubstantially exceed 600 volts, so as to comply with the requirements ofthe fire underwriters, a substantial saving is accomplished in copperrequired, -inasmuch as duplication of winding is avoided. Additionally,corresponding savings in 'physical dimensions of the core are attendantwith possible diminution in over-all dimensions of the core. A neater,smaller, lighter and more compact power unit is thus achieved, withsubstantial savings in iron content.

In Figure 2, I have disclosed the mode of carrying into effect suchautotransformer connection. Therein lead 24 from secondary SI isconnected to lead 2| of primary I8 at junction 29, while lead 21 fromsecondary S2 is connected to lead 2Il of primary I8 at junction 30.Primary current flow is just as has been traced in Figure- 1. Secondarycircuit is established from the left side of secondary SI, lead 25, tube22, lead 24', junction 30, lead 20, primary I8, lead 2I, junction 29,lead 24, and back to the right side of primary SI. For secondary S2, 9,similar current can be traced: From the right side of S2, lead 26,including condenser 28, tube 23, lead 24, junction 29, lead 2I, windingI8, lead 20, junction 30, and lead 21, back to winding S2. For reversalof primary current, during reverse half-cycle, the current flow is justthe opposite to that traced.

- In Figure 3 I have disclosed the relation of the condenser 28 to thetransformer unit in the assembled powerunit, the combination oftransformer and condenser conveniently being referred to as the powerunit. It is entirely possible, in

, final assembly, to contour condenser 28 differently from what is shownin the drawing, and to associate it closely and snugly with thetransformer unit in the interest of small compass, compactness andself-contained comtruction, of pleasing appearance to the purchaser. Thegeneral relation of the several parts of thetransformer of Figure 3 isexactly in conformity with Figure 1, so that no amplification isnecessary.

I have stated hereinbefore that the design of both sets of transformercore shunts is in calibrated conformity with the nature of theparticular tube loads serviced by the corresponding secondaries. Theload of secondary SI, for example, gives rise to a comparatively lowreluctance when the tube 22 is in operation. Accordingly, were theleakage reactance of the associated shunt path to be of appreciablevalue, the primary flux, always seeking the path of minimum reluctance,would still in large measure traverse the main flux path, even when thearc had been struck across tube 22. In nicely calibrated manner,therefore, the reluctance of the air-gaps GI, G2 is closely computed,and the airgaps design accordingly, to provide just the proper ratiounder load conditions between the'quantities of primary flux shuntedacross the leakage paths, and that still traversing the main path.

- 8 In the case of the secondary S2, servicing the power-factorcorrecting condenser and associated tube load 23, the condenser 28interposes a high reactive flux, so that if air-gaps G3, G4 have no morereluctance than gaps GI, G2, they will bleed substantially all theprimary flux when the tube load is energized. Insufficient primary fluxwould course the main circuit to maintain the required are voltage. Theare would extinguish, and the tube would remain de-energized.Accordingly, therefore, the air-gaps G3 and G4 are made substantiallylonger than the air-gaps GI and G2. Moreover, I have found that thewidth of shunts I1, I1 need only be about half as wide as shunts I6, I6,thus permitting a very compact unit which nevertheless is protected inthe event of a short-circuit of condenser 28 or grounding of tube 23. Inaddition, I find a better distribution of flux is had with shunts I], I1being narrower than shunts I6, ii, there apparently being a functionalrelationship between the length of air-gaps and narrowness of theassociated shunts.

It is readily apparent that with the excess of by my new construction,it is possible to diminish the applied line voltage, and still providefor an induced secondary voltage of suflicient value to strike the arc.The only difference would be that the arc would strike later in eachhalf-cycle, and extinguish earlier. Thus, a dimmer operation isavailable, of practical utility.

By the practice of my invention, iron and copper requirements have beenminimized, so that a more compact and self-contained unit has been madepossible, at a substantial saving in initial investment. The newassembly is sturdy and subsequently fool-proof, it being virtuallyimpossible to damage the installation permanently, even uponshort-circuit in both secondaries. Not only the transformer unitconsidered alone, but as well, both the power unit and tube assembly,rigidly comply with all requirements of the Fire Underwriters. Allnecessity of protective auxiliaries, expensive in themselves, has beenfiectively eliminated. No fuse or other circuit protecting device isrequired in the condenser circuit because full protection againstexcessive shortcircuit current is had with the fore-shortened coreshunts. Stroboscopic efiect has been avoided, and good systempower-factor achieved. Long and satisfactory tube has been madepossible, even under cold weather conditions, and quick-strikingcharacteristics have been imparted to the tubes.

All these and many other thoroughly practical and important advantageshave been imparted by the practice of my invention. Since manymodifications may be made of the embodiments which I have disclosed, andsince many embodiments of my basic principles may be evolved, theforegoing description is to be considered as merely illustrative and notby way of limitation.

I claim:

A three-coil transformer comprising a shelltype closed magnetic corehaving a central leg and two outer legs flanking said central leg onopposite sides thereof-and providing a pair of main magnetic flux paths;two pairs of magnetic core shunts, those of one pair being shorter andsubstantially narrower than those of the other and defining threecompartments between said pairs of shunts and providing two pairs ofmagnetic flux paths shunting across said main magnetic flux paths; aprimary coil positioned on said central leg in the middle compartments;and two 10 secondary coils positioned on said central leg, one NumberName Date in each compartment on opposite sides of said 2,212,198 SolaAug. 20, 1940 shunts from that hOllSlIlg' the primary coil. 2,265,700Outt Dec. 9, 1941 JOHN HEROLD BRIDGES. 2,289,175 Boucher July 7, 1942 52,298,935 Freeman Oct. 13, 1942 REFERENCES CITED 2,317,844 Boucher et a1Apr. 27. 1943 The following references are of record in the 23351910Boucher -1 file 33352;; E il" "E t 3? 2132;

q ouc er e a. ar. UNITED STATES PATENTS 10 2,410,624 Boucher Nov. 5,1946 Number Name Date 1,895,231 Pearson et a1. Jan. 24, 1933

